Thanks so much. Turns out the specific issue was that in trying to create an NPN phase splitter to test out the diff Amp, he had ONLY caps on the collector and emitter of the phase splitter. These in turn were connected to the respective bases of the two NPN transistors of the diff Amp.
When he hung the channel 1 scope probe on either of the outputs of the phase splitter, the diff Amp output on channel two "worked", or at least he got an output signal that matched the sine wave input.
When he disconnected the scope probe from the output of the phase splitter, the circuit essentially